Our previous observations on the radiation-induced dissociation and reassociation of polysomes have provided a plausible unifying explanation for many discrepancies in the literature concerning the effect of radiation on protein synthesis. It is now necessary to obtain a biochemical background for understanding the interrelationships between protein synthesis and radiation damage. To this end, we will investigate a number of hypotheses which could explain the postirradiation alterations in polysome content: fluctuations in the rate of initiation, elongation, or termination of protein synthesis; radiation-induced efflux of large nuclear RNA into the cytoplasm to compete with normal messages; a lesion in the endoplasmic reticulum leading to a transient loss of membrane-associated ribosomes; requisite new synthesis of a labile RNA species; and tighter packing of ribosomes on mRNA in polysomes after the recovery. Specifically, we will measure the weight average molecular weight of nascent and released peptides and the contributions of various newly synthesized and old species of RNA to the polysomes by SDS-polyacrylamide gel electrophoresis; the rate of peptide chain elongation by determining the average time for a ribosome to traverse an mRNA; and the rate of initiation as a rate of incorporation of methionine into N-terminal positions by a new assay employing the Edman reagent, phenylisothiocyanate. A detailed understanding of the mechanism of this radiation-induced modification in protein synthesis should provide a conceptual framework for evaluating numerous physiological studies on the roles of protein synthesis in the induction of and/or recovery from radiation-induced mitotic delay, chromosome aberrations, premutational damage and potentially lethal damage, and for manipulating the radiosensitivity of normal and cancerous cells.